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<div class="chapter">
<div class="titlepage"><div>
<div><h2 class="title">
<a name="any"></a>Chapter 4. Boost.Any</h2></div>
<div><div class="author"><h3 class="author">
<span class="firstname">Kevlin</span> <span class="surname">Henney</span>
</h3></div></div>
<div><p class="copyright">Copyright © 2001 Kevlin Henney</p></div>
<div><p class="copyright">Copyright © 2013-2021 Antony Polukhin</p></div>
<div><div class="legalnotice">
<a name="id-1.3.5.1.4"></a><p>Distributed under the Boost Software License, Version 1.0.
      (See accompanying file <code class="filename">LICENSE_1_0.txt</code> or copy at 
      <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
      </p>
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<div class="toc">
<p><b>Table of Contents</b></p>
<dl class="toc">
<dt><span class="section"><a href="any.html#id-1.3.5.3">Introduction</a></span></dt>
<dt><span class="section"><a href="any/s02.html">Examples</a></span></dt>
<dt><span class="section"><a href="any/reference.html">Reference</a></span></dt>
<dd><dl>
<dt><span class="section"><a href="any/reference.html#any.ValueType"><span class="emphasis"><em>ValueType</em></span> requirements</a></span></dt>
<dt><span class="section"><a href="any/reference.html#header.boost.any_hpp">Header &lt;boost/any.hpp&gt;</a></span></dt>
<dt><span class="section"><a href="any/reference.html#header.boost.any.bad_any_cast_hpp">Header &lt;boost/any/bad_any_cast.hpp&gt;</a></span></dt>
<dt><span class="section"><a href="any/reference.html#header.boost.any.basic_any_hpp">Header &lt;boost/any/basic_any.hpp&gt;</a></span></dt>
</dl></dd>
<dt><span class="section"><a href="any/s04.html">Acknowledgements</a></span></dt>
</dl>
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<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="id-1.3.5.3"></a>Introduction</h2></div></div></div>
<p>There are times when a generic (in the sense of
    <span class="emphasis"><em>general</em></span> as opposed to
    <span class="emphasis"><em>template-based programming</em></span>) type is needed:
    variables that are truly variable, accommodating values of many
    other more specific types rather than C++'s normal strict and
    static types. We can distinguish three basic kinds of generic
    type:</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem"><p>Converting types that can hold one of a number of
        possible value types, e.g. <code class="computeroutput">int</code> and
        <code class="computeroutput">string</code>, and freely convert between them, for
        instance interpreting <code class="computeroutput">5</code> as <code class="computeroutput">"5"</code> or
        vice-versa.  Such types are common in scripting and other
        interpreted
        languages. 
        <code class="computeroutput">boost::lexical_cast</code>
        supports such conversion functionality.</p></li>
<li class="listitem"><p>
        Discriminated types that contain values of different types but
        do not attempt conversion between them, i.e. <code class="computeroutput">5</code> is
        held strictly as an <code class="computeroutput">int</code> and is not implicitly
        convertible either to <code class="computeroutput">"5"</code> or to
        <code class="computeroutput">5.0</code>. Their indifference to interpretation but
        awareness of type effectively makes them safe, generic
        containers of single values, with no scope for surprises from
        ambiguous conversions.</p></li>
<li class="listitem"><p>
        Indiscriminate types that can refer to anything but are
        oblivious to the actual underlying type, entrusting all forms
        of access and interpretation to the programmer. This niche is
        dominated by <code class="computeroutput">void *</code>, which offers plenty of scope
        for surprising, undefined behavior.</p></li>
</ul></div>
<p>The <code class="computeroutput"><a class="link" href="boost/any.html" title="Class any">boost::any</a></code> class
    (based on the class of the same name described in <a href="http://www.two-sdg.demon.co.uk/curbralan/papers/ValuedConversions.pdf" target="_top">"Valued
    Conversions"</a> by Kevlin Henney, <span class="emphasis"><em>C++
    Report</em></span> 12(7), July/August 2000) is a variant value type
    based on the second category. It supports copying of any value
    type and safe checked extraction of that value strictly against
    its type. A similar design, offering more appropriate operators,
    can be used for a generalized function adaptor,
    <code class="computeroutput">any_function</code>, a generalized iterator adaptor,
    <code class="computeroutput">any_iterator</code>, and other object types that need
    uniform runtime treatment but support only compile-time template
    parameter conformance.</p>
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